Purification process of resin usable for semiconductor use or optical use is required to remove foreign matters and volatile contents as much as possible to obtain a highly purified resin. In particular, in a case of resin for plastic optical fibers, it is required to reduce scattering and absorption which prevent propagation of light, to an extremely low level.
In order to remove foreign matters from the resin, filtering is the most suitable method. Here, a method of precisely filtering a raw material of resin such as a monomer and polymerizing the raw material to produce an optical resin, has been known. However, there is a wide demand for using resin produced by various methods, for optical use, and it is necessary to remove foreign matters from the resin produced. In order to directly filter resin, it is necessary to filter molten resin. Therefore, a high temperature filtering device durable under high temperatures is required. In the existing techniques, there is no known method to obtain an optically satisfying resin by directly filtering a molten resin. If the resin can be dissolved by a solvent into a solution, foreign matters can be removed by filtering the solution. When the resin solution is filtered, the foreign matters can be removed sufficiently by a known filtering method. However, in a case of the resin for optical use, volatile contents also need to be removed at the same time. This is because the volatile contents cause bubbling at the time of a heat process to the optical resin, or the solvent absorbs light. Various methods for recovering the resin from a resin solution have been proposed. For example, JP-A-8-11835 proposes a method of removing volatile contents after forming a thin layer of resin solution by rotating vanes. JP-A-2000-102921 proposes a method of drying a molten resin by employing a perforated plate. Further, Hosokawa Micron Corporation provides a method having a tradename of “CRUX System”. This resin recovery method is a method of heating a resin solution to a temperature at least the boiling point of the solvent by employing a heating pipe, to evaporate the solvent and further heating the evaporated solvent to form a high speed jet flow of the superheated vapor of solvent, whereby the vapor of solvent and resin content are separated without causing choking of the heating pipe. Further, as a flow control method for highly purified molten resin by which little foreign matters is interfused, JP-A-2001-38725 proposes a control method of changing the temperature to control a flow state of molten resin.
Among the above conventional methods for recovering resin from a resin solution, the method described in JP-A-8-11835 employs an apparatus having a sliding part bearing high speed rotation. However, it was unavoidable to generate dust from the sliding part. Further, in the method described in JP-A-2000-102921 a resin solution of low concentration could not be directly treated on the basis that drying method is for a high viscosity fluid. Namely, the resin solution of low concentration and low viscosity, from which foreign matters should be sufficiently removed by filtering, could not be treated. Further, in the above “CRUX System”, there have been problems in that the method can not reduce the amount of residual solvent to a level required for the optical resin, and in that prevention of contamination is difficult because the resin obtained contains bubbles and foreign matters tend to be involved in the resin when the resin is taken out of the apparatus.
Thus, heretofore, there has not been known any recovery apparatus or recovery method for resin in which foreign matters and volatile contents are removed sufficiently from the resin and interfusion of foreign matters can be prevented and for permitting continuous process. The present invention is to solve the above-mentioned problems and to provide a recovery apparatus and a recovery method for resin suitable for optical resin. Specifically, the present invention has been achieved by finding that contamination resistance can be improved by combining a specific rough drying step and a specific precise drying step and by adding a specific method for controlling the flow of resin in a molten state, and further, by combining the entirety in an organic system.